The central nucleus of the inferior colliculus (CNIC) forms the representation of the acoustic environment that is presented to the auditory thalamocortical system. It does so by combining information from a number of disparate parallel representations in the many brainstem nuclei that project to the CNIC. The proposed experiments will study the nature of the collicular representation of complex natural sounds with the goal of understanding the neural representation in an awake primate preparation, the CNIC of the common marmoset. Aim 1 is designed to resolve questions about the nature of the neural representation of sound that have arisen from studies in rodent and carnivore brainstem and midbrain. There are differences in the degree of inhibitory processing in these preparations which may be related to species differences or to differences in anesthesia. By using an awake primate, we can determine the nature of the brainstem representation in a preparation that is probably closer to human hearing than any of the non-primates studied so far. An important task for the auditory system is to analyze complex acoustic "scenes" to separate different auditory sources, which involves analysis of the mixture of sounds presented to the ear and grouping of different spectral components of sounds into those that originate from the same source. In Aim 2, we will take advantage of the marmoset's vocal repertoire to analyze the representation of complex natural stimuli, marmoset vocalizations, in the presence of backgrounds constructed to be similar to the forest environment in which marmosets live naturally. In Aim 3, we will analyze the representation of temporal aspects of stimuli in the CNIC. Neurons in the CNIC have significant responses to acoustic events occurring over both very short time scales (milliseconds as in the transients associated with syllables in vocalizations) and longer time scales (tens to hundreds of milliseconds as in responses resembling the precedence effect, duration tuning, and other phenomena). We will characterize temporal interactions in marmoset CNIC using an information-theoretic technique that is designed to resolve the temporal "memory" of CNIC neurons.